1. Field of the Invention
The invention relates generally to a method of crystallizing and purifying alkyl gallates and, more specifically, to a method of crystallizing and purifying esters of gallic acid or 3,4,5-trihydroxybenzoic acid.
2. Background of the Prior Art
Gallic acid, or 3,4,5-trihydroxybenzoic acid, is often obtained by acid or alkaline hydrolysis of tannins, which are natural substances, widespread and readily available in the environment. White, T. Of the Science of Food and Agriculture, 1957, 8, 377. Gallic acid can also be obtained via hydrolysis of spent broths from Aspergillus niger or Penicillium glaucum. This trihydroxybenzoic acid can be transferred by esterification with alcohol into the methyl, propyl, or lauryl ester, which are all widely used food and feed antioxidant additives.
Some of these esters can also be found as such in nature. Methyl gallate is one of the biologically active components of Galla Rhois. Ahn, Y.-S.; Kmeon, J.-H. J. of Applied Microbiology, 1998, 84, 439. In addition to their strong antioxidant effects, gallates also exhibit quite powerful antimicrobial activities. Ahn, Y.-S.; Kmeon, J.-H. J. of Applied Microbiology, 1998, 84, 439; Stevens, S. E. J. of Aquatic Animal Health, 1997, 9, 309.
This wide range of biological activities of gallates has generated interest in them for commercial application. Unfortunately, the prior art method of synthesis by esterification of gallic acid, including the steps of purification and crystallization, is rather complex and difficult, with the result that the gallates are expensive to produce and to use.
Although several production methods of relatively pure crystals of alkyl gallates are described in the prior art, all methods for the synthesis of higher alkyl gallates (n&gt;5) are rather impractical for large production purposes, result in a product that is difficult to purify, and/or require toxic and expensive solvents to remove the formed water azeotropically. There is, accordingly, a need for a simple and cost effective general synthesis method for the production of a variety of gallates, particularly higher order gallates, with high purity.